Pressure-gain combustion increases pressure across a combustion chamber thereby thermodynamically approximating a constant volume process, resulting in higher efficiency engines than conventional constant-pressure combustion engines. One method to achieve pressure-gain combustion is with an oscillatory combustion apparatus such as pulse jets or a pulse detonation engine (otherwise known as “pulse detonation combustor”) that carry out pulse detonation combustion.
Pulse detonation combustion is a type of pressure gain combustion process wherein an engine is pulsed to allow a combustible mixture in the combustion chamber to be purged and renewed in between detonations triggered by an ignition source. The detonation is a supersonic combustion event wherein a flame front becomes coupled to a shock wave and propagates through a reactive mixture at sonic velocities. As a consequence, its thermodynamic behaviour effectively approaches that of a constant-volume combustion process which provides higher pressure, higher thermal efficiency and lower specific fuel consumption compared with constant-pressure or steady deflagration processes. Pulse detonation combustors are potentially thermodynamically more efficient because they rely on a pressure rise from a supersonic, shock-induced combustion wave, rather than the constant pressure deflagration process in a standard constant-pressure combustor. The flame speed in a pulse detonation can travel at 6000 fps., compared to 20-70 fps in a conventional constant pressure combustor.
The operational cycle of a single detonation cycle is comprised of filling a detonation tube with a combustible mixture of fuel and oxidant, igniting the mixture, propagating a detonation wave towards the discharge end of the tube, and expelling the combustion products. In an open ended combustion tube, the products are expelled from the tube by rarefaction waves created by a sudden expansion to atmospheric pressure as the detonation wave exits the open end. The cycle can be repeated several times a second.
Rapid transitioning to detonation is desirable to achieve high operating frequencies resulting in higher power output. The deflagration-to-detonation transition (DDT) is where a subsonic deflagration, created using low energy initiation, transitions to a supersonic detonation. The process can be broken down into four phases: (i) mixture ignition, (ii) combustion wave acceleration, (iii) formation of explosion centres, and (iv) development of the detonation front. The distance and time necessary for transition to detonation is called the run-up distance and time, respectively. Stages (i) to (iii) take up the majority of the total run-up DDT distance and time. The majority of the time for DDT is consumed largely by the laminar to turbulent flame transition. The distance for DDT is more sensitive to the acceleration of the turbulent flame. Obstacles along the flow path such as Shchelkin spirals are known to decrease DDT by shortening the distance and time for stages (ii) and (iii). It is thus desirable to provide a pulse detonation combustor which achieves high operating frequencies for better efficiency and performance. Particularly, it is desirable to provide a pulse detonation combustor which has a reduced total run-up DDT distance and time, thereby enabling high operating frequencies and corresponding improved combustor performance and higher power density.
Another challenge to efficient and effective operation of pulse detonation combustors is controlling combustion product backflow and backpressure caused by detonation shockwaves. One known approach to preventing backflow is to use a mechanical valving system. In pulse detonation combustors with such valving systems, a mechanical valve opens to fill a detonation chamber with a combustible mixture and closes thereafter during the detonation initiation and propagation stages as well as the blowdown stages. Exemplary valving mechanisms are described in U.S. Pat. No. 7,621,118 and U.S. Pat. No. 6,505,462. These valving mechanism impose mechanical complexity and tend to be prone to mechanical and thermal fatigue issues that lead to limited service life and additional service maintenance requirements. The operational frequency of the apparatus can also be limited by a mechanical valving system.